This invention relates to a semiconductor device and, more particularly, to an improvement on a contact resistance characteristic between an electrode layer and a semiconductor layer.
With the recent trend for reducing the size and increasing the integration density of integrated circuit elements, great importance has been attached to techniques of forming a conductive layer, e.g., a silicon layer, in contact holes. As a technique for forming a silicon layer in a contact hole, a selective epitaxial growth process has heretofore been used.
FIG. 1 is a sectional view showing a prior art MOS transistor. In the Figure, reference numeral 1 designates a semiconductor substrate of p-conductivity type; numeral 2 is a gate insulation film; numeral 3, a gate electrode; numeral 4, a field oxide film, numeral 5, a non-doped CVD (chemical vapor deposition) SiO.sub.2 (silicon dioxide) film which is formed by CVD process and is not doped with an impurity; numeral 6, a BPSG (boro-phospho-silicate glass) film; numeral 7, a single crystal silicon layer of n-conductivity type; and numerals 8 and 9 are source and drain layers of an n-conductivity type. Numerals 10 and 11 designate respective source and drain electrodes made of aluminum. Silicon layer 7 is formed by the epitaxial growth process. A feature of this structure resides in that BPSG film 6 and n-type silicon layer 7 are not in contact with each other. In the MOS transistor of the above structure, however, the contact resistance between n-type silicon layer 7 and each of aluminum lead layers 10 and 11 on silicon layer 7 fluctuates greatly among individual MOS transistors.
In the above circumstances, it has been desired to reduce fluctuations of the contact resistance between silicon layer 7 and each of aluminum lead layers 10 and 11 with individual elements
Also, with the above structure the contact resistance between n-type silicon layer 7 and each aluminum lead layer is high, so that non-ohmic contact characteristics result. More specifically, to provide for ohmic contact characteristics of the contact between n-type silicon layer 7 and the aluminum lead layer by impurity concentration of silicon layer 7 has to be set to be 1.times.10.sup.20 cm.sup.-3 or above. In order to increase the impurity concentration of a silicon layer formed in a contact hole by the selective epitaxial growth process, it is necessary to increase the concentration of the doping gas, e.g., PH.sub.3, AsH.sub.3 or B.sub.2 H.sub.6. However, increasing the concentration of the doping gas leads to a different problem in that the characteristics of the silicon layer growth are changed. Thus, it has been desired to provide for ohmic contact characteristics of the contact between silicon layer 7 and the aluminum lead layer by reducing the contact resistance therebetween without changing the growth characteristic of silicon layer 7.
Further, while ohmic contact characteristics are obtained between a p-type silicon layer and an aluminum lead layer, the operation speed can be further increased by reducing the contact resistance. For this reason, it has been desired to reduce the contact resistance in the case where a p-type silicon layer is used.